Micro Electro Mechanical Systems (MEMS) technology has enabled the development of numerous small actuators and sensors, which have found use across myriad applications. Due, in part, to the fabrication technologies employed to fabricate these devices, the bulk of MEMS actuators developed have been “in-plane” devices. An in-plane device is one whose response is along a direction parallel to the substrate on which it is formed. The development of devices that operate “out of plane,” particularly actuators, has been limited. Those “vertical” actuators that have been developed typically have had a limited range of motion and, as a result, widespread adoption of vertical actuators remains unrealized.
In many applications, however, vertical actuation or force generation can provide significant advantages, such as smaller required chip real estate, an ability to form large arrays, and broader operational capability. Pop-up, fold-out, and deformable structures are seen as particularly attractive in applications such as tunable RF devices, rotatable mirrors for redirecting light beams, tactile feedback systems, and deformable mirrors for adaptive optics.
Typically, vertical actuation has relied on either electrostatic or thermal actuators, such as vertical comb drives or thermal bimorph elements. A vertical comb drive includes two sets of electrodes that partially interleave and are vertically offset from one another, where one of the sets is operatively coupled with a movable element. When a voltage is applied between the sets of electrodes, a high electric field is generated and the movable set of electrodes attempts to move into vertical alignment with the stationary set thereby giving rise to motion of the movable element.
A thermal actuator typically includes a bimorph structure that has elements of different materials with different thermal expansion coefficients. One end of the thermal actuator is normally free to move relative to the other end. Upon heating or cooling, the different expansion of the materials gives rise to a bending force along the length of the bimorph moving the free end away from or toward the substrate on which they are formed.
Unfortunately, in some applications, it is undesirable to generate high electric fields or regions of localized heat. In biomedical applications, for example, sensitive tissue can be damaged or destroyed by heat or high electric fields. Further, conventional MEMS technology is often difficult to employ in a biological application because of material incompatibilities—particularly for implantable devices that must operate in-vivo for extended periods of time. As a result, it is necessary to package such devices to limit the exposure of the MEMS materials to the biological environment and visa-versa. Such packaging increases overall cost and can degrade device performance in many cases.